01_readme.md

Examples for my metaprogramming/DSL talk

02_method_generator.rb

module MyAttrAccessor
  def my_attr_accessor(attr_name)
    my_attr_reader(attr_name)
    my_attr_writer(attr_name)
  end

  def my_attr_reader(attr_name)
    instance_variable_name = attr_to_variable_name(attr_name)

    define_method(attr_name) do
      instance_variable_get(instance_variable_name)
    end
  end

  def my_attr_writer(attr_name)
    instance_variable_name = attr_to_variable_name(attr_name)

    define_method("#{attr_name}=") do |value|
      instance_variable_set(instance_variable_name, value)
    end
  end

  private

  def attr_to_variable_name(attr_name)
    "@#{attr_name}"
  end
end

Object.extend(MyAttrAccessor)

class User
  my_attr_accessor :email

  def initialize(email: nil)
    @email = email
  end
end

user = User.new
user.email = 'john.doe@example.com'
puts user.email # => john.doe@example.com

03_module_generator.rb

module MyAttrAccessor
  def self.[](attr_name)
    Module.new do
      instance_variable_name = "@#{attr_name}"

      define_method(attr_name) do
        instance_variable_get(instance_variable_name)
      end

      define_method("#{attr_name}=") do |value|
        instance_variable_set(instance_variable_name, value)
      end
    end
  end
end

class User
  include MyAttrAccessor[:email]

  def initialize(email: nil)
    @email = email
  end
end

user = User.new
user.email = 'john.doe@example.com'
puts user.email # => john.doe@example.com

04_builder_simple.rb

class Builder
  def get(path, **args)
    routes[['get', path]] = args
  end

  def post(path, **args)
    routes[['post', path]] = args
  end

  def routes
    @routes ||= {}
  end
end

class Router
  NOT_FOUND = 'NOT_FOUND'

  def self.draw
    builder = Builder.new
    yield(builder)
    new(builder)
  end

  def initialize(builder)
    @builder = builder
  end

  def route(method_and_path)
    method, path = method_and_path.split(' ')
    @builder.routes[[method.downcase, path]] || NOT_FOUND
  end
end

router = Router.draw do |builder|
  builder.get '/users', controller: :users, action: :index
  builder.post '/users', controller: :users, action: :create
end

puts router.route('GET /users') # => {:controller=>:users, :action=>:index}
puts router.route('POST /users') # => {:controller=>:users, :action=>:create}
puts router.route('GET /orders') # => NOT_FOUND

05_builder_instance_eval.rb

class Router
  NOT_FOUND = 'NOT_FOUND'

  def self.draw(&block)
    new.tap { |router| router.instance_eval(&block) }
  end

  def route(method_and_path)
    method, path = method_and_path.split(' ')
    routes[[method.downcase, path]] || NOT_FOUND
  end

  def get(path, **args)
    routes[['get', path]] = args
  end

  def post(path, **args)
    routes[['post', path]] = args
  end

  def routes
    @routes ||= {}
  end
end

router = Router.draw do
  get '/users', controller: :users, action: :index
  post '/users', controller: :users, action: :create
end

puts router.route('GET /users') # => {:controller=>:users, :action=>:index}
puts router.route('POST /users') # => {:controller=>:users, :action=>:create}
puts router.route('GET /orders') # => NOT_FOUND

06_builder_generate_methods.rb

class Router
  class << self
    def draw(&block)
      new.tap { |router| router.instance_eval(&block) }
    end

    def generate_route_methods(*method_names)
      method_names.each do |method_name|
        define_method(method_name) do |path, **args|
          routes[[method_name, path]] = args
        end
      end
    end
  end

  generate_route_methods :get, :post, :patch, :delete

  NOT_FOUND = 'NOT_FOUND'

  def route(method_and_path)
    method, path = method_and_path.split(' ')
    routes[[method.downcase, path]] || NOT_FOUND
  end

  def routes
    @routes ||= {}
  end
end

router = Router.draw do
  get '/users', controller: :users, action: :index
  post '/users', controller: :users, action: :create
end

puts router.route('GET /users') # => {:controller=>:users, :action=>:index}
puts router.route('POST /users') # => {:controller=>:users, :action=>:create}
puts router.route('GET /orders') # => NOT_FOUND

07_instance_and_class_methods.rb

class DB
  def self.instance
    @instance ||= new
  end

  def users
    @users ||= {}
  end
end

module CoolRecord
  def self.included(base)
    base.extend(ClassMethods)
  end

  def save
    record_id =
      if new_record?
        self.id = (DB.instance.users.map(&:id).max || 0) + 1
      else
        id
      end

    self.class.table[record_id] = { id: record_id, email: email, name: name }
  end

  def new_record?
    id.nil?
  end

  module ClassMethods
    def where(conditions)
      table.values
        .select { |attrs| conditions.all? { |(attr_name, value)| attrs[attr_name] == value } }
        .map { |attrs| self.new(**attrs) }
    end

    def find_by(conditions)
      where(conditions).first
    end

    def table
      DB.instance.public_send(table_name)
    end

    private

    def table_name
      "#{self.name.downcase}s"
    end
  end
end

class User
  include CoolRecord

  attr_accessor :id, :email, :name

  def initialize(id: nil, email: nil, name: nil)
    @id = id
    @email = email
    @name = name
  end
end

user = User.new(email: 'john@example.com', name: 'John')
user.save

puts User.where(name: 'John').inspect # => [#<User:0x00007fa5f385c2f8 @id=1, @email="john@example.com", @name="John">]
puts User.find_by(name: 'John').inspect # => #<User:0x00007fa5f3834398 @id=1, @email="john@example.com", @name="John">
puts User.where(name: 'Jane').inspect # => []

08_method_generator_class_eval.rb

module MyAttrAccessor
  def my_attr_accessor(attr_name)
    class_eval <<~TEMPLATE
      def #{attr_name}
        @#{attr_name}
      end

      def #{attr_name}=value
        @#{attr_name} = value
      end
    TEMPLATE
  end
end

Object.extend(MyAttrAccessor)

class User
  my_attr_accessor :email

  def initialize(email: nil)
    @email = email
  end
end

user = User.new
user.email = 'john.doe@example.com'
puts user.email # => john.doe@example.com